In technologies of encapsulating a semiconductor, transfer molding previously has been the mainstream. The transfer molding, in which resin flowing occurs, involves various problems such as a limit on the package size, adverse influences of thinner and denser package size on chips and wires, warpage of package due to inhomogeneous distribution of fillers in molding, and inefficient use of resin due to the necessity of cull and runner. Based on these backgrounds, application of compression molding methods has been investigated, and various encapsulation materials in a sheet form also have been investigated not only in a liquid form (Patent Literatures 1, 2).
By the compression molding method, encapsulation can be performed without serious problems at present when a small-diameter wafer or a small-diameter substrate made of a metal and so on of approximately 200 mm (8 inches) is used, but there has been a serious problem that the substrate or the wafer warps due to shrinkage stress of an epoxy resin and the like at the time of the encapsulating and curing when encapsulating a large-diameter substrate having semiconductor devices mounted thereon or a large-diameter wafer having semiconductor devices formed thereon with a size of 300 mm (12 inches) or above. Furthermore, when encapsulating a semiconductor devices mounting surface of the large-diameter substrate having the semiconductor devices mounted thereon on a wafer level, there arises a problem that the semiconductor devices peel away from the substrate of a metal and the like due to the shrinkage stress of the epoxy resin and so on at the time of the encapsulating and curing, and that the next operation cannot be performed to cause a serious problem that mass production is impossible.
A method for solving the problems involved by an increase in diameter of the substrate having the semiconductor devices mounted thereon or the wafer having the semiconductor devices formed thereon as described above is to fill an encapsulating resin composition with nearly 90 wt % of a filler or reduce the shrinkage stress at the time of curing based on reduction of the elasticity of the encapsulating resin composition (Patent Literatures 3, 4, 5).
However, there newly arises a problem that, when filling with nearly 90 wt % of the filler, viscosity of the encapsulating resin composition increases, and the encapsulating resin composition applies force to the semiconductor devices mounted on the substrate at the time of casting, molding, and encapsulating, whereby the semiconductor devices peels away from the substrate, which is the same problem as in the transfer molding. Moreover, when the elasticity of the cured material of the encapsulating resin is lowered, the warp of the encapsulated substrate having the semiconductor devices mounted thereon or the encapsulated wafer having the semiconductor devices formed thereon can be improved, but a reduction in encapsulating performance such as heat resistance or moisture resistance newly occurs. Therefore, these methods cannot solve the problem fundamentally.
Accordingly, there has been demanded an encapsulant which can collectively encapsulate a semiconductor devices mounting surface of a substrate having the semiconductor devices mounted thereon or a semiconductor devices forming surface of a wafer having semiconductor devices formed thereon on a wafer level without causing warpage of the substrate or the wafer or peeling of the semiconductor devices from the substrate made of a metal and so on even though the large-diameter wafer or large-diameter substrate made of a metal and the like is encapsulated to attain excellent encapsulating performance such as heat resistance or moisture resistance after the encapsulation.
To solve these problems, a covering methods has been reported in which a semiconductor devices mounting surface of a substrate having the semiconductor devices mounted thereon or a semiconductor devices forming surface of a wafer having semiconductor devices formed thereon is covered with a fiber substrate impregnated with a thermosetting resin in which the fiber substrate is impregnated with the thermosetting resin and the thermosetting resin is fully cured, together with an uncured thermosetting resin composition formed on one surface of the fiber substrate impregnated with a thermosetting resin. This method reduces the warpage after molding by making the expansion coefficients of the fiber substrate and the semiconductor devices mounting substrate closer. Actually, it is very difficult to mount an uncured thermosetting resin composition onto a fiber base material, and is hard to attain handleability after the mounting.